text:
Can BOTH instrument rated pilot under the hood and safety pilot log PIC time in IFR flight?
This question is somehow related to question: Can a non-instrument rated safety pilot log PIC during an IFR flight? , but the scenario is different.
Scenario
Pilot A and Pilot B are BOTH instrument rated. Pilot A wants to fly under the hood to practice and Pilot B will be the safety pilot. They will be flying in an IFR flight, in VMC conditions.
Question
Can both pilots log PIC time? Pilot A would be logging PIC time for being the sole manipulator of the controls, and Pilot B would only log PIC as a required crewmember/safety pilot when Pilot A was under the hood.

Fly what you want...log what you need.

I believe the key to this scenario is in the FAA Counsel Office's interpretation to Speranza (2009)
Quoting from the FAA's interpretation letter:

The FAA has previously stated that there is a distinction between logging PIC time and acting as a PIC. See Herman Interpretation. To act as a PIC (i.e., the pilot who has final authority and responsibility for the operation and safety of the flight), a pilot must be properly rated in the aircraft and be properly rated and authorized to conduct the flight. In your example of an IFR flight, being properly rated and authorized would include having an instrument rating. Accordingly, only Pilot A may act as the PIC, and Pilot A has final authority and responsibility for the safety of the flight regardless of who is manipulating the controls.

They also clarify the circumstances under which Pilot B may log PIC time for this flight, and what that means for Pilot B logging PIC time:

Accordingly, Pilot B may log PIC time for the portion of the flight during which Pilot B was the sole manipulator of the controls.
  . . .
  However, Pilot A may not log PIC time for the portion of the flight during which Pilot B is the sole manipulator ofthe controls, and is logging PIC time, because there is no provision for this logging in §61.51(e). Section 61.51(e)(1)(iii) allows the pilot acting as PIC to log PIC time only if more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted, and only one pilot is required for the flight in your example. Although Pilot B properly may log PIC time for a portion of the flight as discussed previously, Pilot B could not act as PIC and was not a required flight crewmember for any portion of the flight under the aircraft's type certificate or the regulations under which the flight was conducted.

(Emphasis mine in both quotations.)

So, because the flight is conducted under instrument flight rules you must hold an instrument rating (be "properly rated and authorized" to conduct the flight) in order to act as PIC and log PIC time on that basis, regardless of the weather conditions (VMC/IMC) under which the flight is conducted.
A key difference between your case and the one in the interpretation is that both of your pilots have instrument ratings. Since the safety pilot in your example holds an instrument rating they are rated for the aircraft and authorized to conduct a flight under Instrument Flight Rules, therefore they can act as PIC, and would be eligible to log PIC time for this flight (as a required crewmember under FAR 91.109).
This would, as best I can tell, revert to the same logging situation as a safety pilot under VFR: For any portion of the flight in which the safety pilot is also manipulating the flight controls the pilot under the hood would be unable to log PIC time, and for any portion of the flight where "Pilot A" is not under the hood they safety pilot would (generally) be unable to log PIC time.

I agree with your interpretation of the reg but the specific question asked says Pilot A and Pilot B are BOTH instrument rated.  Perhaps edit your TLDR slightly?

@SteveV. Actually I misread that part of the question, and that may change my interpretation of the regs: 91.109 requires a safety pilot any time you're operating in "simulated IMC", 61.51(e)(1)(iii) says a safety pilot can act as PIC as provided they're rated (and per the interpretation, authorized) to conduct the flight -- both of those conditions would appear to apply if the safety pilot is instrument-rated, so in absence of an interpretation from the FAA to the contrary I think in this case both pilots could log PIC time & have updated accordingly.

Short version: yes, but only if the pilots agree in advance that the safety pilot is acting PIC.
When to log PIC
Per 14 CFR 61.51 there are only three ways - ignoring training scenarios - to log PIC (my emphasis):

(i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;
(ii) When the pilot is the sole occupant in the aircraft;
(iii) When the pilot, except for a holder of a sport or recreational
  pilot certificate, acts as pilot in command of an aircraft for which
  more than one pilot is required under the type certification of the
  aircraft or the regulations under which the flight is conducted; or

In your scenario, both pilots are qualified to operate the aircraft under IFR, and two pilots are required for simulated IMC by regulation. Therefore both the first and third reasons are applicable, the question is how to 'match' them to the pilots and the key question there is: who is acting PIC?
Option 1: the safety pilot is acting PIC
If the safety pilot is acting PIC then he can log PIC time under point 3 because the regulations require two pilots for simulated IMC. And, since the 'pilot flying' (to borrow an airline term) is the one physically flying the aircraft, he can also log PIC as the sole manipulator of the controls.
Option 2: the pilot flying is acting PIC
If the 'pilot flying' is acting PIC then he can log PIC time under point 3 as described above. However the safety pilot can't: he isn't acting PIC, and he isn't manipulating the controls. In this case, the safety pilot can only log SIC time per 61.51(f)(2):

(f) Logging second-in-command flight time. A person may log
  second-in-command time only for that flight time during which that
  person:
[...]
(2) Holds the appropriate category, class, and instrument rating (if
  an instrument rating is required for the flight) for the aircraft
  being flown, and more than one pilot is required under the type
  certification of the aircraft or the regulations under which the
  flight is being conducted.

Of course, if the safety pilot ever takes the controls, then he can log PIC for that time under the first reason.
PIC responsibility
This might all seem like playing with words, and two friends going for a flight might not even discuss explicitly who's acting PIC, although they should. 14 CFR 91.3 says:

The pilot in command of an aircraft is directly responsible for, and
  is the final authority as to, the operation of that aircraft.

What that means for this scenario is that if something goes wrong during the flight or a regulation is broken, the FAA will ask who was the acting PIC. If the pilots agreed that the safety pilot was PIC then the flight is his responsibility and he can't say that "I was just the lookout". Possibly even worse, if the pilots don't know who was acting PIC, then the FAA will give both of them a hard time.

They can both log PIC in VMC, IFR flight regardless of whether one or both of them are IFR current or rated also regardless of which one of them is flying the simulated instrument and which one is acting as the safety pilot.
The accepted answer in the linked question is wrong.
It is the same whether the safety pilot is instrument rated or not as long as the other pilot is.
The IFR current pilot is acting as PIC regarding the IFR duties and flight control manipulating duties of the flight.  The safety pilot is acting as the PIC handling the see and avoid duties.
Simulated IMC is invalid in actual IMC conditions, therefore this question would not apply in that situation.
This was actually linked above the other, incorrect, yet accepted, answer in a comment PILOT COUNSEL: SAFETY PILOT
WHEN YOU NEED SOMEONE BESIDE YOU

"If the pilot under the hood is instrument rated and acting as pilot in
  command, even though the flight is IFR, the safety pilot is not
  required to have an instrument rating or be instrument current."

Hi Ryan.  I'm going to disagree with you.   You say: They can both log PIC in VMC, IFR flight regardless of whether one or both of them are IFR current or rated also regardless of which one of them is flying the simulated instrument and which one is acting as the safety pilot.
Suppose Instrument rated Irene and Private pilot Paul go flying. Irene gets her IFR clearance, puts on a hood, and shoots three approaches. Paul never touches the controls and only watches for traffic.  Where in 61.51(e)(1)(i) or 61.51(e)(1)(iii) allows Paul to log PIC?

@SteveV. my interpretation would be the last part of 61.51(e)(1)(iii), "...the regulations under which the flight is conducted."  You make a great point, but the thing is, if this were true during IFR that a safety pilot may not log PIC, it would also appear that a safety pilot could never log PIC ever not even under visual flight rules, but it is clearly legal to do so under VFR.  You are right though; the regs are pretty much a bare skeleton on this matter.

Your interpretation seems to differ from the FAA Counsel Office's interpretation - To act as a PIC (i.e., the pilot who has final authority and responsibility for the operation and safety of the flight), a pilot must be properly rated in the aircraft and be properly rated and authorized to conduct the flight. In your example of an IFR flight, being properly rated and authorized would include having an instrument rating.

Specifically, my read of that interpretation is that a flight conducted under IFR (even in VMC) requires that the person logging "acting as PIC" (responsible for the operation & safety of the flight - which is what gives a safety pilot the ability to log PIC time) would require an instrument rating to be "authorized to conduct the flight" -- you can't (legally) operate an IFR flight without an instrument rating, even in VMC. If the flight were conducted VFR with "practice approaches" then both pilots could log PIC as they're both properly rated & authorized to conduct the flight.

I completely agree with voretaq7 here (and Steve V.).  In this situation, the non-instrument rated safety pilot could log flight time since he is a required crewmember, however he could not log PIC time because he is unqualified to be the PIC on an IFR flight.  This would leave him logging SIC time. If the flight was operated under VFR, then he could indeed choose to be the PIC (assuming that he is otherwise qualified) and let his friend fly with a view limiting device and they could both log it as PIC time.

Your last quoted sentence is absolutely correct, but doesn't go against what we are saying.  The safety pilot is not required to have an instrument, however, it doesn't mention that he is unable to log PIC in that situation even though he is legal to be the safety pilot.

@Lnafziger If the flight is operated under VFR then could the pilot under the hood log it as instrument time?

@TomMcW Absolutely. That is the intent of this rule: to allow a pilot to maintain instrument currency even when they haven't been able to fly in IMC conditions.

@Lnafziger To log an IAP if you're not on IFR doesn't someone have to simulate the vectors and clearances? The FAA gives an example of an instructor simulating IFR ATC instructions on a VFR flight, but if the safety pilot is not IFR rated could they do this? Or if you ask ATC for practice approaches will they do the same thing they do for an IFR flight minus the clearances?

@TomMcW You have several options: you could do as you suggested, or just fly the full approach from an IAF and not even need vectors.


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Why does the Boeing 747 have a nickname but other aircraft do not?
Boeing 747 is commonly called Jumbo Jet.
How did it get this nickname and why other airplanes do not have nicknames?

Please note that I am not talking about official names, e.g, Boeing 787 Dreamliner.

My local ATC calls the A380 a super Airbus

Isn't the 787 also known as the Dreamliner?

@JayCarr The full name is Boeing 787 Dreamliner. It is not a nickname.

The B-52 is the BUFF, the EMB-145 is the "jungle jet".  They all have nicknames and google can tell you all of them.  I have more commonly heard the 747 called a "whale" than a "jumbo jet".

Swearingen Metroliners used to be referred to as Lawn Darts and also as San Antonio Sewer Pipes. Aeronca Champions were referred to as Air Knockers. F-105s were called Thuds in reference to the number of them that "thudded" into a ridge near Hanoi that was appropriately named Thud Ridge and gave rise to a book of the same name. 737s were for a time called Fat Albert.
I was always under the impression that the 747 got the Jumbo Jet nickname simply because when it first came out, it was the so uniquely bigger than anything else in passenger service at the time.

Two aircraft models were known as The Widowmaker during their operational lifetimes: the WW2 Martin B-26 Maurader and in West Germany during the cold war days the Lockheed F-104.

Here is a fairly extensive list: http://www.b737.org.uk/aircraftnicknames.htm

@segfault: Might be because aircraft are classified by weight (as proxy for wake turbulence for purpose of separation) to ‘light’, ‘medium’, ‘heavy’ and ‘super’ and the last category was created specifically for A380. And in many places (certainly in USA), pilots of the ‘heavy’ and ‘super’ are required to add that designation to their callsign (so controllers are always aware of the required separation).

@casey Among the general public, I've never heard the 747 called anything other than a "jumbo".

"Jumbo jet" refers to all widebody (twin-aisle) jetliners, not just the 747.

How Boeing 747 is called Jumbo Jet?

The Boeing 747 is the world's first wide-body aircraft. For a long time, it was the world's largest passenger aircraft1. When Boeing was designing the 747 in mid-1960s, it expected that in the near future supersonic travel would become the norm. Hence, there would not be a need for the passenger version of 747. Because of this, 747s were designed in a way that they can be easily converted into freighters.
Pan Am, being one of the first customers of 747, wanted to fit two rows of cargo containers2, instead of one. This meant that the fuselage had to be at least 16 feet wide, wider than any airliner at that time. This would have worked fine for cargo, but it would make loading and unloading passengers very time-consuming. To accommodate this, Boeing engineers designed two aisles, which ran parallel through the length of the aircraft, hence the term wide-body was coined.
When the press noticed the huge size, they described it with the catchy term Jumbo Jet. It is also known as Queen of the Skies but that is not common.

Do other airplanes have nicknames?

Airbus A380 dubbed as superjumbo by media. The only A380 sold as a business jet is called Flying Palace.
Although there aren't official nicknames of airplanes, almost all airplanes earned some nick names. This list contains nicknames for mostly military aircraft.

1 Airbus 380 took over as world's largest passenger airplane in 2007.
2 At that time, cargo containers were cubes that measured 8 feet (2.44 m) per side.

Didn't the 747 start as a competitor to the Lockheed C-5? Boeing had the government fund the early development, and the raised cockpit still bears witness of that today. But they also wanted their military airlifter to be converted to a passenger airplane and chose a low wing design. To load that, you need to fold up the nose and move the cockpit out of the way. Lockheed went for a purely military-oriented layout with high wings and a rear ramp, similar to the C-141, and won the competition.

@PeterKämpf: Reading the wikipedia article I get the impression that it was kind of both. They had military requirements and they had PanAm requirements and they kinda matched enough so that they could reuse much of development.

a lot of aircraft have official nicknames, especially military ones. The term "dreamliner" for the 787 was coined by Boeing's marketing department for example. Many aircraft even have more than one nickname, like the A-10 which is officially called "Thunderbolt II" but known to its crews as "Warthog" or simply "Hog".

The "jumbo jet" moniker isn't exclusive to the 747, but is applied to any large commercial jet - the L-1011 and DC-10 (if I'm recalling the model names correctly) were also called "jumbo jets", as is the large Airbus. It's just that the 747 was much more successful than its early competitors.
Other aircraft which are called "jumbo jets" are listed here.

I find it interesting that Wikipedia says " 'Jumbo jet' redirects here" on the Wide-body aircraft article and " 'Jumbo Jet' redirects here" on the Boeing 747 article. Apparently "Jumbo Jet" was indeed a nickname of the 747 before the term started being used to refer to any wide-body aircraft. And, yes, L-1011 and DC-10 were both also wide-bodies that competed with the 747, but the 747 was first. The DC-10 was actually fairly successful selling. It sold 445 through 1988 vs. 711 for the B747.

@reirab: Still, "jumbo jet" always was a generic term for a large passenger aircraft.  It's just more common to see it used WRT the 747 because a) it was the first one; and b) there are a lot more of them flying.  I don't think either the DC-10 or L-1011 are still used for passenger service.

Yeah, they're not. They're just used for cargo these days, when they're used at all.

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What are the flight training options for a private pilot's license in the US?
In the USA, what training options does a student pilot aiming for their private pilot license have?

There are lots of options. Flight training is the US is usually divided into part 61 and part 141. Part 61 is general flight instruction and any FAA approved instructor can teach you under it: many part 61 instructors are independent or work for or with FBOs at GA airports. Part 141 governs flight schools that have a standard curriculum and are much more structured; they're often associated with training commercial pilots looking for airline careers. The license you get is exactly the same whichever way you learn.
There are pros and cons to both approaches, this article summarizes them well but the most important general point is probably that part 61 is 'one on one' training so it can be tailored for you and fit your schedule, whereas part 141 is 'school' training and if you can't follow their schedule then it usually isn't possible anyway. For that reason, part 61 is often the only practical option for many private pilots who have jobs and families.
Perhaps your real question here is "how do I find an instructor?", in which case you need to search the web, check out local FBOs and flight schools, ask for recommendations from pilots you know and so on. It's definitely worth investing some time to find the right instructor for you, because that can make a huge difference to your progress (and your costs).

One other difference between part 61 and part 141 is that the required minimum total hours for part 141 is 35 whereas for part 61 it is 40 hours. For most students this is a moot point because most students need more than the minimum number of hours to both master the skills and complete the various requirements.

Practically, the biggest variance in cost is going to be the price per hour of the airplane you rent.
A brand new Garmin 1000 equipped 172 might be 175 bucks an hour plus.  There may or may not be a fuel surcharge on top of that.  A 1968 Piper Warrior with basic instruments might be 100 an hour fuel included.
For basic VFR training you don't need a shiny new airplane with lots of bells and whistles, so if you're shopping for price, shop based on the price and availability of rental planes.  Instructor fees and other costs won't vary much.

Wow, no wonder so many people go to the US for flying lessons. I paid $300 (AUD) an hour for lessons in a beat up old C152 here in Australia. Basically about double the price. I loved it, but the cost is a killer.

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text:
What is the market size of high-pressure turbine blades for commercial aircraft?
I need to calculate the market size for high pressure turbine blades which are used in commercial aircraft. I am working on a business school project on this topic.
Any information which can be used to calculate the total size will be useful:

Key players and their market size
Market trends
Growth
Latest innovation
Latest news
Pricing strategy of these blades by companies
Merger and acquisitions
Customer info
Any key factors to consider

What research have you done so far?

GE, P&W, RR, and their joint ventures (CFM, EA) own the designs (intellectual property) and either manufacture them themselves or under an exclusive sourcing agreement. They don't sell individual parts. They sell bundled MRO services to operators with their engines.  Without their cooperation you wouldn't be able to build an approved part and they have no incentive to do so.

Airbus market study http://www.airbus.com/content/dam/corporate-topics/publications/backgrounders/Airbus_Global_Market_Forecast_2017-2036_Growing_Horizons_full_book.pdf gives their estimate of the market over the next 20 years. Boeing does the same thing, their numbers are similar.
2 engines per aircraft, roughly.
Rolls Royce Trent XWB has 68 HPT blades per engine. https://www.rolls-royce.com/products-and-services/civil-aerospace/airlines/trent-xwb.aspx#latest-updates Use that as a guide.
As for cost of blade I don't know, its of the order of thousands of $ per blade


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How can you find AGL if the altimeter is set to MSL?
Without doing the math, how do you know your true altitude AGL when your altimeter is set to MSL

The math isn't that difficult: subtract the field elevation

Are you asking about an aircraft in flight or sitting on the ground? See this question too.

I usually don't care a lot about my AGL altitude.
When I'm doing my flight planning on the ground, I do take into account terrain and obstacles, and pick appropriate MSL altitudes to be well clear of anything on the ground.
But by the time I get in my plane, I'm thinking in terms of MSL (altimeter) and no longer about AGL.
That even applies to approaching my destination field.  If I know the field is at 1,500 ft. and I want to do a traffic pattern at 1,000ft. AGL, then I plan for my altimeter to read 2,500 (MSL).  And I think about if I'm at the proper MSL altitude (2,500), not what my AGL altitude is.


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What is the purpose of drag and anti drag wires?
What are drag and anti drag wires,and how important are they to the performance and strength of the wings of an airplane?

http://www.stearman.net/index.php/drag-and-anti-drag-wireswhy-are-they-in-the-wrong-place/ Helpful link I found though there are no images.

Very helpful article...appreciate! @ironduke97........

So we are supposed to believe that the wings are making thrust, not drag?  Something seems wrong with this picture.

It's hard to believe that that would happen in any part of the flight envelope. Is the point that at high angles-of-attack the force generated by the wings acts forward in the aircraft's reference frame, yet still acts aft in relation to the direction of the flight path?

Definition from the Stearman Restorers Association (emphasis added):

Drag wires run from a point inboard and forward in a bay to a point outboard and aft.  In this position, the Drag wire is there to RESIST drag forces that would tend to pull the wing tips aft as the airplane travels through the air.  Anti-Drag Wires oppose the drag wires running from an inboard aft location to an outboard forward point in its bay.  As the name indicates, these anti-drag wires oppose forces in the reverse to the direction of flight.

In a wing without a structural skin like on a fabric covered aircraft, they provide rigidity in the horizontal plane by creating a wire braced truss system of the spars, longitudinal tubes called compression tubes spaced at intervals between the spars and holding them apart, and the drag anti-drag wires forming an X between each pair of compression tubes.
The drag/anti-drag loads are not that high so fairly small wires can be used.  An Aeronca Champ's fabric wing uses about 1/8" steel rods for the job. Some homebuilts just use 1/8 steel control cable.  The wires at the root see the most load.
You can also use diagonal tubes that just run from one corner to the other of each bay and take both compression and tension loads.  Wires are generally a bit lighter.
For your ultralight, bracing could probably be made from 3/32" cable with thimbles and nicropress fittings at the attachments.
You can see the drag/anti-drag wires in this Goat glider.  The plans are open source so there is probably a lot you could copy from this design.

There are several wires in the photo. You might specify which are drag/anti drag.

A better photo will be needed to show the difference too...

Diagram added with red dots indicating the D/AD wires

Mind if I ask....what's the name of the first airplane pic @John K

Sorry David I don't know it was just a random image in Google images.

Try pulling 1/8 inch steel cable apart.  Steel is very good in tension. (wood is outstanding in compression).  The purpose of antidrag wires is to maintain tension in the drag wires. They pull against each other, creating more rigidity or stiffness in the structure.
This type of construction, along with thin undercambered biplane configuration, is not as common these days, stressed skinned cantilever monoplane designs are most popular.
However, especially for the aviation history enthusiast who does not need to go particularly fast, loves gentle landings, and flying with half the horsepower, they are worth looking into.
One caveat is not to go too far downwind with them, it will take you a while to get home.  Checking wire tension as part of pre-flight will help as well.

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text:
How can filling a tube with foam help prevent buckling?
How can filling the empennage (aluminium tube) with Styrofoam stiffen it to avoid buckling as seen in Whingding ultralight?

Sort of like water in a hose prevents it from bending as easily as an empty one. It can still bend/kink if there's enough force, but it takes a lot more force than when just an empty tube.

@SnakeDoc - that's an answer, not a comment!

A simplified analysis of column buckling shows that the axial load required for buckling is directly proportional to the second moment of area of the column's cross section: $F=\pi^2EI/(KL)^2`$`, where F is the buckling load, E is the elastic modulus of the material, I is the second moment of area, and KL is an effective column length based on the geometric length of the column (L) and a variable factor K that is based on the boundary conditions.  The critical term for this question, though, is I.
For example, look at the picture below of a beam that someone is trying to bend.  You can do this yourself with a ruler or a paperback book.

In the upper image, you're trying to bend the book/ruler by pressing down on the spine, and it's really hard to do.  Rotate the book 90 degrees and press down on the cover...and it's relatively easy (unless you picked a phone book).  The material of the book didn't change, only how the area of the cross section that you were bending was distributed in the direction that you were trying to bend the book.  In the same way, adding foam to the tube increases the total area that needs to bend in order for the tube to buckle, which, if you'll allow an approximation, increases the strength of the tube by 5% from the baseline case (a hollow, aluminum tube).  Double the radius, and that becomes 10%.  Double it again, and that goes to around 25%.  Don't take those numbers as gospel, but it should give you an idea that adding area -- even with a relatively low stiffness material -- you can increase the strength of a material significantly.
However, buckling is different from strength.  Buckling is a bending instability caused by compressive loads, and failures in buckling often occur well below the failure load for the structure.  Let's run some numbers for this, using the expression I mentioned back up at the top.
For example, if I run an analysis for a 4.5 ft long, hollow, 1 in diameter tube with a wall thickness of 1/16 of an inch made out of 7075-T6 aluminum (E = 10.3 msi) with both ends effectively pinned, I get a buckling load of 714 lbf (note: the axial compressive load that would make this structure yield would be closer to 8,000 lbf).  Naturally, one could fill that in with metal and increase buckling load dramatically to 1,727 lbf, and, while that's not the query, I am adding some bending stiffness to the tube by filling in the center.  Buckling of a composite column (insofar as I know) is tricky, but, here is an approximation (which will undershoot the actual buckling strength).
If I add foam and say that the foam will not buckle (as it is fully constrained by the tube and cannot deform in any direction save axially, let us say), and then assume also that the purely axial load on the column will be divided between the foam and the aluminum according to the proportion of their elastic moduli (polystyrene has a Young's modulus of about 2.5 GPa), then the aluminum will only absorb about 88% of the load placed on the column.  This increases the buckling load by about 100 lbf (without causing the foam to yield).  However, this does not take into account how the foam will add bending stiffness to the structure, which would further increase the benefits of adding the foam.
In short, you're using a low-weight, cohesive filler material to increase the bending stiffness of a structure, dramatically increasing its tolerance for compressive loads.

So the Styrofoam must be in one piece inside of the tube @Marius

For best results.  If it is not in one piece, it will reinforce the tube around those pieces, but the interface between the two will become a weak point in the tube.  I wouldn't say that it will have the same buckling strength at that point as a non-reinforced tube because of the interactions between the two pieces of foam that meet there, but it will not be anywhere as strong as the reinforced tube because of the lack of continuity in the reinforcement.

@Marius Have you got a number for if the tube were solid aluminum, rather than hollow or filled with foam? The contribution from the foam surprised me, and I'm wondering about the original strength of the aluminum.

@aerobot - Actually, I should revise my answer.  The 1,727 would be for a solid aluminum rod.  That said, styrofoam will increase the buckling load, but I couldn't tell you exactly by how much because it greatly complicates the phenomenon.  If I make an assumption that the load divides according to the young's moduli of each material, and that the foam won't buckle because it's surrounded by the tube, I can show an increase in buckling load by about 100 lbf.  Good catch.

That's the simplified analysis, huh....

Solid aluminum may also be heavier than foam filled aluminum... so long as it meets the engineering requirements, you'd probably go with the lighter of the two options.

@SnakeDoc - yes, it absolutely would be--I'm just using it as an example for how buckling load changes if you vary the second area moment of a hollow tube.  That said, I would reference the last sentence in my answer to echo what you're saying: the advantage of adding foam is that you're using a lightweight filler material to make a significant and positive impact on the structural integrity of this part.

A simple explanation of buckling failure is a kinking of the tube wall. Force on the tube deforms it out of round. A flat side is weaker than a curved side just as a flat piece of paper bends more easily than a paper tube. With enough force the flat side folds.
Filling the tube with any material that prevents the initial deformation (keeps the tube round) will strengthen the tube. Solid, rigid foam does this and it is lightweight, important for an aircraft.
You could also seal the ends of the tube and inflate it with air, using the pressure of the air to keep the tube round. Commercial airliner fuselages are strengthened in flight this way.

Nice answer, +1 as a motivation to write more of them. If you were a bit earlier you could have saved me some time.

Yes he would have ....thank you for the super simplified answer@pilothead

First of all, lets start with a short introduction on buckling. There are two types of buckling, global (or column) buckling, and local buckling.
Global buckling is what happens to long slender (thin) structures, with a compressive load. Say you take a piece of plastic pipe with a diameter of 3/4 inch (2 cm +/-) and a length of 8 feet (2.5 m +/-). You place it vertically on the floor, and you start pushing on it. If it is very straight, you have to push it quite hard, and then it suddenly curves in to a hoop, and you need very little force to push it further. Foam in the wing helps very little against this type of buckling because the increase in stiffness is very small because of the low stiffness of the foam. That also does not matter, because there is no compressive load empennage at all, therefore this type cannot occur there.
Local buckling happens to thin-walled structures, when the thickness is small compared to the diameter. It can happen due to compressive loads, bending or torsion (external pressure can also make it buckle or buckle faster). This is what happens when you crush a cola can. Dents start to form and the structure loses its strength. You can do some interesting experiments with this at home. First take an empty can without any dents, place it on the ground, and start to crush it with your hands. You will notice that it is very hard to crush the can, you may not even succeed (I once put a 35 kg block of steel on an empty can easily). Now take an empty can and make a few big dents in a can and try to crush it again, it will be very easy (with a long stick I made a dent in the can with the block of steel on it, and it fell down so hard the whole building was shaking. I had to explain to my colleagues that i was giving a demonstration on buckling). Now try to crush a full can of cola, I doubt you will get there. The pressure of the cola is making sure that the can cannot dent, therefore it cannot buckle.
The cola in the can is doing the same thing as the foam in the wing; it makes sure that the empennage cannot dent (maybe cannot is an overstatement, but it sure helps!), and therefore it will not buckle.

Wow.....thank you so much sir.......or ma! @rick boender


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How do you load test a wing rib?
When builders say they load tested a wing, most of the time what they are saying is that they load tested the wing spar(s) by turning the plane upside down and putting weights on the wing. This procedure doesn't load test the ribs, so how do you load test a single rib?

It seems like loading the wing would load up the whole wing, not just the spars.

What country are you asking about? As far as I know, there is no requirement to test single ribs, just the assembly as a whole. Testing a single rib may be meaningless anyway since it obtains (and contributes) strength in regards to the entire structure, not just the single structural member.

@RonBeyer based on OP's track record, they are asking for the purposes of a homebuilt ultralight, and their country is Nigeria.

Testing a rib by itself may not give proper results. If the rib is kept straight as by a metal skin when assembled into a wing, testing without the skin will cause a failure at much lower load.

@DavidTeahay I'd say proctoring it the SAT or GED would give it considerable amounts of stress, so if you do that you'll have to be careful it doesn't crack. If it does well, then good luck to your new wing going off to a wonderful university! :P

Rib is load tested upside down with weights on its flat bottom side to simulate air deflection during flight,and two ropes holding the rib up at positions where the spars will sit.


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How is it possible to have 102% indicated?
Watching some videos, I noticed that an sr22 was showing 102% power on takeoff. How is this possible?

What is "your plane" in this context? What are you looking at to see "102% power"? An N1 indicator? (Questions already address that.) Something else? Very unclear.

Just in general. I've seen it in sr22 and turbines with n1.

I watched some videos where the Cirrus sr22 was showing 102% power on takeoff. Why is this?

Because the engine was running at 102% power, I guess. Is your question "How is it possible to exceed 100%?"

Yes, I don't understand how that's possible.

Then you should edit your question to ask how it's possible to exceed 100%, and maybe it will be reopened.

If you are wondering how this is possible you may want to reference a similar example here:  https://www.youtube.com/watch?v=hW008FcKr3Q

If you are hired to work 40 hours/week, but one week you work 10 hours of overtime, i.e. a total of 50 hours/week, then how many % of your contractually agreed time have you worked that week?

"100%" is a relative number and its not uncommon in aircraft engines to see higher than "100%" operations published. Often "100%" is the maximum continuous operation setting or some other relative power limitation. Manufacturers will test and certify their engines with other limitations like "102% for XX minutes" or similar limitations. This means you can run the engine at some given (high) power setting for a fixed amount of time safely but not indefinitely at that setting.


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Why doesn’t ACARS just use HF if it has better range than VHF?
Why does ACARS default to VHF if HF has a better range?
For example, when requesting ATIS from an airport, it will first try VHF, and if that fails, it will try HF.  And if all hope seems lost, it will try Satcom. Why is this?

The audio with HF is generally awful even under good conditions. It's always the last resort if other means are available.

If you are outside of VHF range you don’t need ATIS.

@MichaelHall  Not necessarily. The altitudes & (close in) waypoints of a STAR often depend on which flow an airport is using, so it's very helpful to know that "ATL is landing east" pretty far out so that you can set up, verify, and brief everything in the FMS as far out as possible.

@JohnK  That's true, but the question is about ACARS data rather than voice communication.  Of course, the things that make audio quality so awful on HF also reduce its bandwidth, as the accepted answer explains.

Ahh yes I didn't read it carefully.

@RalphJ, I agree getting ATIS as far out as possible is helpful, and I have tried and been unable until being closer, my comment was an admittedly slightly snarky jab at the overkill of using a transoceanic capable radio to get terminal information.

The main concern is that radio spectrum is a shared resource, meaning that while you’re using it, nobody else within the range of your transmission can use it too. That means you always want to use a shorter-range band whenever possible.
Another is that HF channels are significantly narrower (lower data rate) than VHF ones, so not only are you consuming a more scarce resource, you’re also consuming it for longer periods.


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Why is it necessary to put in the destination airport elevation for the pressurization system?
Why do pilots put the destination elevation into the flight computer for the pressurization?

Pressurization control.

Related: How Cabin Pressures Are Selected?

See How does the cabin pressure controller work in a large commercial airliner?
When cruising, the pressure in the plane is greater than outside, and it's not  just a fixed amount, nor does it just start pressurization at a particular altitude (like 8000 feet).   Instead the pressure system tries to slowly pressurize and depressurize to minimize discomfort.
If the aircraft didn't know the destination altitude, then after landing you might have to either do a fast decompression, or wait longer than you wanted before opening the doors.  This way, the the system can plan the equalization at gentle rates and have it complete shortly after the aircraft reaches the target altitude.

At cruise altitude, there is a differential between the cabin pressure and the outside pressure.  At landing, you want that differential to be (roughly) zero.  In the descent from cruise to landing, that differential decreases toward zero.
(In practice, you actually land with a slight positive differential, which dissipates shortly after weight-on-wheels.  So not exactly zero differential at landing, but close to it.)
The schedule for decreasing that differential to zero will be different for an aircraft cruising at FL 370 and descending to land at sea level, than it will for an aircraft starting at the same altitude and descending to land at 5,000' MSL.  In the former case, there will still be some differential pressure at 5,000'; in the latter case, it will be down to about zero.
Knowing the landing field elevation allows the pressurization controller to schedule the reduction in differential pressure so that it reaches zero at the right time (altitude) in the descent.
If the aircraft landing at sea level had zero differential at 5,000' (i.e. followed the schedule for landing in, say, Denver), then passengers would feel the full aircraft rate of descent the rest of the way down, which could be a lot greater than you'd prefer -- i.e. 1,500 FPM (typical for a clean 250 knot descent) vs a less ear-popping 300-500 FPM (typical for cabin rate of climb/descent).
On the flip side, if the aircraft landing in Denver used the schedule for the aircraft landing at sea level, it would land with some differential pressure still on the aircraft, which isn't desirable either.  Since a pressurized cabin makes it hard or (with enough differential) impossible to open the exits, the aircraft systems will typically vent any remaining cabin pressure shortly after touchdown, meaning that if significant differential exists, the cabin altitude would then CLIMB fairly rapidly -- again, uncomfortable for the passengers and their ears and possibly sinuses.  (Alternatively, if the aircraft doesn't vent the pressures, then opening the exits can have some problems.)
The pressurization controller can sense the altitude of the departure airfield and is given both the cruise altitude and the landing altitude, allowing it to schedule the cabin pressure differential going both up and down.  Without the landing field elevation, it wouldn't be able to do the latter.


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How did the propellers on the B-29 stop perfectly in sync?
In the photos below, the propellers on the B-29 props all stop in the same position. Is this done by hand, it is this automatic?

Hmm, I suppose vertical props would pose a reduced chance of hitting one's head.

Note that the planes visible in the background of the Enola Gay picture do not each have their propellers all in the same orientation.

With the 18 cylinder radial engines in the B-29, a random shutdown would give possible propeller positions ever twenty degrees.  They do not automatically stop this way.
However, for staged photos like these, where the appearance of the aircraft is important to those commissioning the image, ground crews would hand rotate the propellers into alignment -- and this was just as much the case in 1944 as it is today.
Notice that the photo of Enola Gay, despite being on a remote airfield, is staged enough that the photographer used a red filter to darken the sky and show the clouds dramatically.  This isn't something that was done for just any photograph, because it required several times as much exposure (meaning the camera was most likely placed on a tripod, unlike a common press photo shot hand held).
Per comments, it was also considered a matter of pride, back in the day, to always rotate the propellers to matching positions whenever the airplane wasn't being turned around immediately for another mission.  This is in the same category as always having boots polished so the sergeant can see himself, or a bunk made up so tightly a quarter would bounce on the blanket -- operationally unnecessary, but still required.

An “Old Timer” once told me it should be done every time, not just for photos or inspections. It was a matter of pride.

If I'm not mistaken there are other aircrafts visible in the background of the Enola Gay which do not have their rotors aligned.

Comments are not for extended discussion; this conversation has been moved to chat.

The props are rotated by hand and are all positioned the same way as follows.
For 4-bladed props: when they are pointing "square" (straight up and down and horizontal) it means the engines have been turned over by hand to work the oil out of the bottom cylinders (i.e., ready to crank for startup). For 3-bladed props they leave the bottom-most blade pointing straight downwards to indicate ready to crank.
If the blades on all the engines are randomly askew it means they have not been turned over by hand and must be thus exercised before cranking to work the pooled oil out, and prevent blowing a head gasket via a hydraulic lock.

One of the pilots of “Doc”, the CAF B29 in the picture, sometimes talks about “indexing” the propellers from the cockpit when ground clearance is needed.  For instance, when they taxi at a smaller general aviation airport, they have to shutdown the outboard engines before they can leave the runway.  There isn’t enough ground clearance under the B29 props (18”) to not hit the taxiway lights.  Before leaving the runway he shuts down the outboard engines and manually “indexes” the propellers using the starter to put them in the X orientation.
This is a consequence of tricycle gear.  The B17, for instance, is a tail dragger and thus the props are higher off the ground during taxi.

Nice answer (I know I'm not supposed to say that but I did anyway.)

But-- I'm having a little trouble with the geometry. Would the prop blades really be at risk of hitting the taxi lights?

@quietflyer Yes.  Take Midway for instance.  The taxiway letter boxes are over 24" tall and spaced about 85' apart.  The B29 has 18" of prop clearance and a wingspan of 141'.  It would be really close.  At my airport, a midsized corporate reliver under a metro Class B, the boxes are 60' apart, there's no way they could make it through the outboard food processors.  Oshkosh taxiways are 50.  Also, in northern climes our edge lighting is on 18 to 24" standards for visibility when snow is on the ground.  Martin Pauley's youtube channel has a video of a CAF flight engineer taking about it as well.

Is that really necessary? Have you ever landed at Midway?

It's definitely done by hand afterwards.  The ultimate proof I guess would be a video of the props stopping, out of alignment, and then a video of folks aligning them later, or just a photo taken afterwards with the props all aligned.  I don't have that.  But I know there was no mechanism in the B-29 for keeping the props all exactly aligned in flight, so it wouldn't make sense that they would ever stop all in alignment.
Plus, even if hypothetically there was some mechanism to keep the props exactly aligned in flight, it would make no sense that they would always happen to stop in the most aesthetically pleasing orientations ("+" or "x").

The "ultimate proof" is in background of the OP's pic of the Enola Gay.

@FreeMan maybe they were rotating and the 1/1000 s shutter speed made them look like they are stopped, but out of phase (possible if different engines are at different rpms ... right? ). (I found your comment right, but I am still playing devil's advocate here, for fun and to keep the brain exercised by challenging own assumptions)

Maybe, @EarlGrey, but most people don't shoot a still life at 1/1000.

Having all the props in perfect sync would be bad for the airframe - each time a blade passes the leading edge of the wing there would be a slight change in air pressure, adding to vibrations and affecting lift over that section of wing.  Worst case, that's 4 engines x ~16 foot of wing = 64 foot of wing that experiences pressure changes, or just under half its 141 foot wingspan.  Random would be smoother.

@Criggie And this is why Mr. Monk flunked out of aircraft design school and had to become a detective instead...

Not only are the props not necessarily synced in flight, they're not necessarily even turning the same speed--each of the engines has its own, separate throttle.

@JerryCoffin -- just curious -- 1) can you give a few examples, and 2) does the pilot also intentionally run the inboard and outboard engines at different rpms, or just different manifold pressures?

@quietflyer: I edited that part out (before you replied), specifically because I don't remember the details.

@JerryCoffin -- sounds maybe like the basis for a new question to ASE, might get some interesting replies --


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Why do only turboprops have ground/flight idle selections?
Why is it that only turboprops have a pilot controlled flight/ground idle and not jets?

Jets can have multiple "idle" settings, although the distinction generally isn't "flight idle" vs "ground idle".
For a jet aircraft, you usually want idle to be the minimum fuel consumption possible, so you want the slowest possible idle RPM for a long descent (min fuel consumption), and also for ground operations (min thrust -- not riding the brakes during taxi, plus minimize fuel burn).  In certain cases, though, you want a slightly higher power setting, even when the thrust levers are closed against the idle (aft) stop.

When using significant amounts of bleed air for anti-icing or de-icing would be one case where a FADEC (or a less-automated predecessor system) would command a slightly higher idle.
Some aircraft may have an "approach idle" where the min power setting while on approach -- defined however, perhaps in terms of altitude and/or configuration -- is kept higher than otherwise so that power is more immediately available in the event of a go-around.

For a turboprop, you're typically running at constant propeller RPM, and the difference between high power and low power is a matter of the blade angle.  Flight idle would keep the RPM high but the blade angle flat -- taking a minimal "bite" out of the air, doing little work, generating minimal or no thrust, and having the blades moderately "flat" in reference to the oncoming airflow.  But, the prop is still rotating at the nominal speed in terms of RPM.  On the ground, the requirements change, and you can accept (or may find desirable) an even flatter blade angle and a lower RPM than you would in flight.  Thus, the "ground idle" option that accomplishes those things, burning less fuel (since the prop is spinning more slowly and is taking even less bit out of the air, generating no thrust) and generating less noise.
In both the jet and the turboprop case, you can have different idle settings, but they are there to accomplish different purposes, driven by the presence or absence of the propeller and its constant RPM & varying pitch.  In the jet, the switch between idle modes is generally transparent to the pilot; you know which one applies "now" by noting your N1 RPM when at idle.  In the turboprop, ground idle is typically selected by the pilot.

Why would a pilot need flight idle instead of ground idle while in-flight? Aren't the economic and flight requirements identical, meaning a ground idle with lower RPM and "even flatter" blade angle is advantageous, especially when go-around power is more easily achievable in a turboprop?

Flat blades become a major source of drag - known as "going into Beta range" right after landing. The lowest blade angle you need in flight is Flight Idle; the really flat blades & much slower engine speed (from which spool-up would take significant time) is reserved for ground use - thus, Ground Idle.


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Are IAPs designed around an average barometric pressure for an airport?
When implementing IAPs, is atmospheric conditions taken into consideration? For example, for any given airport there will be varying atmospheric pressure. This will slightly affect the DH and DA. How might an airport take a wide range of altimeter settings in consideration?

The barometric pressure at the airport doesn't matter as long as you are using that airport's altimeter setting. The altimeter setting is chosen so that it indicates the correct altitude MSL when at field elevation, so regardless of the barometric pressure at field elevation, it will indicate the correct altitude MSL when you are approaching the field.
The temperature can cause additional altimeter errors, however. When it is very cold, the altimeter at field elevation will still indicate field elevation, but the altimeter will indicate that your are higher than you actually are when you are above field elevation. This is why certain instrument approach procedures are not allowed when the outside air temperature is too low, and others require adjustments to the minimums.
The temperature being too high is not a problem from a safety standpoint- the altimeter will indicate lower than it should, but that just means that you are farther from obstacles than you would be on a cooler day.


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Why (and how) do aircraft rudders center themselves without input?
Why (and how) do almost all aircraft rudders have some sort of way of re-centering the rudder when there's no input being made? For example, if you wanted to move the flight controls (fly by wire aside) you would move it to either the left or right and most of the time it will stay in that position.
Is this in order to prevent Dutch roll and other oscillations?
Edit- what ways can be implemented to provide this centering? For example I know on the Boeing 737 there is a self-centering mechanism that centers the flight controls. Do all planes have some sort of centering for the rudder's, alierons, and the elevators?

The reason rudder surfaces are self-centering is to provide hands-off stability, where the natural tendency for the control surface is to adopt a zero-deflection state in the absence of inputs from the pilot. This prevents the plane from flying itself suddenly out-of-control (which is called divergent response) if the pilot has to momentarily let go of the yoke or rudder pedals for any reason.
The rudder is made self-centering by design during flight by placing its hinge axis ahead of its aerodynamic center of pressure.

Is this a by-design feature or just a by-nature aspect of the control surface? Your response implies that there's a mechanism or something about the design that causes this, while Chris' answer says that it's the nature of control surfaces.

In flight, I would expect it to self-center due to the airflow, but not on the ground. If there is no self-centering device, maybe it's just the orientation of the rudder, plus gravity?

@William, will edit.

@Steve, will edit.

@user253751, will edit.

The flight controls are connected directly to the flight surfaces. The flight surface will naturally settle to a point of least resistance- i.e. whatever they are trimmed to. The pilot has to actively apply pressure in order to hold the flight surfaces away from the trim point, and when the pressure is released they will naturally return to the trim points.
This applies to all the flight controls- if you pull back on the yoke and then let go, it will definitely not stay in the same place.

A backward swept hinge line on the vertical tail means that the center of gravity of the rudder will be lowest in the center position, so the rudder will self-center on the ground by gravity alone. Still, it is desirable to tailor the force gradient of any control surface such that there is a detent at the center position to give force-feedback to the pilot when the control surface is in its center position. This is achieved in flight sometimes by a thick or blunt trailing edge. The fixed trim tabs on the rudder of a Klemm 35 below (own work) serve the same purpose.

Mechanical detents are normally found on trottle or flap levers. Primary control surfaces are mostly free-floating, with the exception of the control column in Bombardier aircraft where there is a noticeable detent in pitch direction. Bombardier calls this the "sneeze protection".
Ailerons need mass balancing to prevent flutter. On the ground, an overbalanced aileron with differential linkage (more trailing-edge up movement than down) will always fall into the fully-deflected position due to gravity. The balancing mass on the trailing-up side will have a larger lever arm and pull the opposite side with it.
In flight the aerodynamic force will center the control surface at the trim position (which might not be the center) - that is what trim tabs or trim springs are for. But this only means that there is a zero-moment point in an otherwise smooth moment gradient over deflection angle. Some aerodynamic tricks help to increase the gradient locally at the zero point (see the Klemm rudder above), but those are not generally adopted. The smooth gradient, on the other hand, is a certification requirement.

So it's more of an aerodynamic reason vs having an actual self centering mechanism?

@Boeing787 In flight: Yes. but it need not settle in the center position. In a sideslip it will settle at the floating angle.


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Why minutes of latitude tick marks point up and and those of longitude point left

As you can see , the minute tick marks point up and left. Why is this?

I cannot see. Could you annotate your image so we're sure we're looking at the same thing?

Why did they pick a convention of "up" and "left" instead of some other convention? Is that what's being asked?

It may have to do with how our visual perception has been conditioned.  For example, page 11 of the chart users guide for mountain shadows says this: "Shaded relief shows how terrain may appear from the air. Shadows are shown as if light is coming from the northwest, because studies have shown that our visual perception has been conditioned to this view."

@RalphJ Yes that is the question.

Try changing your question title to, "Why minutes of latitude tick marks point up and and those of longitude point left ..." Otherwise your question reads as though they are pointing northwest. It took me some time to understand your question.

That chart is western hemisphere and north of the equator, right?  South of the equator and in the eastern hemisphere you'll see them in the other direction - this is helpful near the boundaries where the numeric values are similar (±0° or ±180°).

These lines are called the graticule. Sectional charts are equivalent of ICAO 1:500,000 VFR charts.
ICAO convention
For ICAO, ticks are defined in Annex 4 to the Chicago convention:

16.5.3 The graduation marks at 1' and 5' intervals shall extend away from the Greenwich Meridian and from the Equator.
Each 10' interval shall be shown by a mark on both sides of the graticule line.

For ICAO, the location of the 1' and 5' ticks varies with the lat/lon quadrant. For a location in the SE quadrant, away means bottom and right. E.g. this ICAO VFR chart for Cairns area in Australia, by 19°30'S, 146°30'W:

Source: Airservices Australia
That also works in the NE quadrant, e.g. for an ICAO VFR chart of France, away is top and right. But France has a small part of Brittany extending to the other side of the reference meridian (NW quadrant). This extension still uses the NE quadrant convention for some reason, which might be practical.
US sectional charts
US sectional charts are the equivalent of ICAO 1:500,000, but don't particularly follow ICAO conventions. Differences between local rules and ICAO recommendations are listed, ordered by annex number, in the US AIP at section GEN 1.7. There are many differences for Annex 4, but not for paragraph 16.5.3 (§16.5.3.1 has differences, but it is related to the size of the ticks, not their placement). So the US sectional charts follow the placement rule ("away") recommended by ICAO. CONUS area is in the NW quadrant, away means top and left.

However I suspect this placement is consistent with ICAO by chance. This allows for the ticks to appear on the white background at the top-left corner when a line is coincident with the corner coordinates:

The alternate choice would put the ticks on the white background at the bottom and on the right side, which is less natural.

So its more of a chart design trick as opposed to an actual useful purpose?

@Boeing787: Yes, a choice had to be made.

@Boeing787, no, it's not a "trick", it serves an "actual useful purpose" by reminding you what hemisphere you are in!

I mean, even if it had no specific use, it's just a standard.  Standards are often arbitrary because you need to have some consistent way to do things no matter what that way is.


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How did novice pilots hit the Twin Towers?
I know people don't like discussing this, and I don't ask out of morbid curiousity.  But conspiracy theories will naturally arise about such great and consequential events, and I don't think it's fair to dismiss all them out of hand without establishing some sort of proof.
For the record, I am certain that the World Trade Center, including the infamous Building 7, was not brought down by controlled demolitions. And not because I believe some kind of "official" version, but because the main premises of that theory are demonstrably false.
But getting the the question:
How did novice pilots with only a few hours of simulator time on the type, manage to maneuver the sluggish, overspeeding jetliners do precisely?
You could argue that their task was much simpler than landing, where you had to precisely control speed, pitch, and altitude and to approach the runway from a specific heading only.
But the aircraft were moving 3 times as fast. Their targets were about as wideb as a runway. They did not have the rotation of the runway to let them know when they were drifting off course. Moreover, I've heard that the hijackers didn't even practice landings.
Yet AA11 hit with a precision of 3ft from center, and within 4° of structural south according to NIST. And UA175, which was sorely off course seconds before impact, still hit with a precision of 30ft of center, and 13° of structural north according to NIST, or 3° according to another source.
How did the 9/11 hijackers find their way to New York City? doesn't answer the question, since navigating to a city is a different challenge from aiming for a building. In the firmer case, the target is much bigger, the time scale is much greater, you can rely on autopilot and digital navigation aids.

There is no reason to think they did not train for much longer than just a few hours and not hundreds, possibly gaining everything that available simulator can give. Regular pilots do not do this kind of task so if any more specifics is involved may miss at the first try.

Granted I don't fly jets, but it's not that hard to fly in the direction you want to go. I've had students that I'm sure could hit a 200ft wide building without any previous experience.

Where have you heard these "anecdotal reports"?

World Trade Center 1 and 2 were pretty big.  That is likely one of the reasons the terrorists chose them as targets.
I think the question should be: How could you miss a 60 m by 400 m target?
For comparison, the touchdown zone of an aircraft carrier is around 15 meters long, and it's approached at an angle of around 10 degrees.   That means experts frequently land fast jets on a target that is 3 meters (sin(10)*15) tall and 7 meters wide without flaring and in the dark.

Yet AA11 hit with a precision of 3ft from center

Maybe they were not aiming for the center.  An evil person might guess that striking off center would cause more of the tower to fall on surrounding buildings, increasing the damage.
They struck the 96th floor out of 110 floors.  Probably they would have done more damage if they struck further down.  A logical evil person would aim near the 60th floor (flying lower might hit another building when approaching from the west), so probably they missed by 36 floors.

I have heard anecdotal accounts of experienced pilots failing to replicate the attack on the World Trade Center.

The attackers did not want to replicate the attack accurately.  They just wanted to cause some spectacular damage.  That's easier.

Striking lower would have caused less damage, because the columns were stronger. Striking higher would trap less people to be smothered by the smoke. And U do rbsee how "they aimed off center" is a valid argument. Rather, it makes the question deeper, since it makes it look like UA175 hit with precision

@sophit The attackers could not have replicated the attack because it didn't happen yet.  Therefore, they did not want to replicate it (they had other reasons they didn't need to be precise).  Conspiracy theorists have a misguided idea that they should try to simulate the attack exactly because the attack had already happened.  If the attack had not happened yet, the conspiracy theorists would not have that wrong idea.  Is that clearer?

No sorry  I interpret the quoted sentence as: "after the 9/11, the hit was reproduced in a flight simulator by experienced pilots and they were not able to hit the twin towers". So I'd expect as an answer to this quote either a "yes this is true" or "no they actually reproduced the hit without any problem"

I think Occam's razor would say that they aimed for the center of the building. It's how you instinctively aim for anything of any size. It's much more difficult to aim precisely for a specific distance from center. And it would require that there was a bunch of structural analysis done ahead of time to determine the best exact point to hit the structure. Not inconceivable, but just seems less likely. The simpler explanation is more plausible, they just wanted to hit the building, possibly with a general idea of how high up they wanted to aim, i.e. halfway up, upper third, near the top etc.

Aircraft carrier glideslope is 3.5 degrees, not 10.

@StevePemberton Mohamed Atta was an architect.  I think it's quite likely he did some kind of analysis.

@AP - my thinking is that he knew there would be some amount of challenge for them in just hitting the towers (my opinion, others here have a different viewpoint). Only if he thought they could hit an exact x-y point would he have selected precise targets. My attempt to express this within the 600 character limit was by adding the qualifier "a bunch of". If you remove "a bunch of" from my comment it would read that I didn't think there would be any structural analysis. By adding "a bunch of" I was trying to say that I thought there would be some analysis, but only related to general height.

We’ve had this discussion before on this forum.
Does this answer your question?
How did the 9/11 hijackers find their way to New York City?
From an operational standpoint, crashing an airplane into a building does not require a whole lot of skill. It probably does take a lot of determination and resolve, knowing you’re about to die.
The skills required to maneuver, an aircraft and hit a target the size of a skyscraper.  I would argue that 90% of all private pilots with no experience in large jets, could, in a Level D full motion, simulator, deliberately pilot the jet into such a building on the first try. As I stated in the comments, it’s simply getting the target on the nose of the airplane and holding there such that it’s not translating in your field of view.  Maintaining altitude would not be that difficult of a chore for these guys either.
And if you missed the target, you can simply circle back and try it again!
When I was a young child, a neighborhood friend and I used to mess around on an old copy of Microsoft Flight Simulator, where we would intentionally crash the airplane into the Sears Tower after departing Meigs field in Chicago.  This was in 1984, mind you, when the American public would have thought the events of 9–11 sounded like the plot to a bad Jerry Bruckheimer movie.  At least, in the realm of that simulation, intentionally colliding a airplane into a building is not a difficult task, even for someone who had no formal flight training!

Navigating to a city is a whole different thing from aiming for a building.

Did you ever hear the phrase “look where you want to go”?  It’s not that difficult.  Point the nose of the aircraft directly at the building, chances are, it’s gonna fly right there (not counting for winds).  As to the concerns about over speeding - I’m gonna fly this airplane ride into a building. Why would I care about over speed warnings?

Are you sure you aren't conflating "simple" with "easy"?

The link that you provided addresses the skills required to navigate to the towers which that question was about, but it said nothing about how easy a task it would be to hit the buildings once they navigated to them. It would help if you provided some reasoning for the assertion that hitting a 200 foot wide object with an airliner travelling over 400 mph (500 mph in the case of UA175) does not require much skill. Clearly whatever skill it required they had, but that doesn't necessarily mean that it was easy. For an airline pilot yes, the question is how much experience would likely be needed.

@StevePemberton You can try it in any freely available flight sim. To put it in simple terms, it's something almost everyone can get on the first try. Big jets pretty much just fly wherever you point them.

@Therac - Experimenting in consumer level flight sim programs is not proof of how easy it would be in real life. That doesn't mean that I don't think it is a valid point in the discussion, however as originally answered no reasons were given, which is what I was replying to. The answer has been edited since then with at least some amount of explanation.

@StevePemberton short answer, it is very easy to fly a large jet like that into a building. We’re not talking about a V1 cut in strong gusting crosswinds or a LOCI or some other emergency requiring a trained pilot.  The people making these kinds of comments that are required an airline pilot with thousands of hours to pull that off are so full of crap they could pass for an outhouse.  Again, conspiracy nitwits play on the ignorance of the public to how flight operations really work in order to leave them astray. I can’t say anything else. If you’re not willing to believe that, good luck to you

I didn't say I didn't believe it, I said you didn't make a case for your argument, all you originally said was that it doesn't take much skill to hit a building flying at 500 mph and didn't give any reason why, only pointing to another thread that didn't even address this topic. You have since edited your post with at least some explanation, so now people have a chance to either agree or disagree with your points about how much experience it would require. And I don't think anyone said that it requires thousands of hours of airline experience, at least not on this thread.

Your reason is very simple. Simply select a point on the building that you want to hit.  Use roll inputs to counter any kind of lateral deflection in flight path from it, and pitch inputs to control your altitude.  It doesn’t require an extreme amount of finesse or high degree of piloting technique to pull this off. And anybody telling you otherwise does not know what you’re talking about. Good luck to you.

@Therac I can't. I have retinitis pigmentosa


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What is the lift to drag ratio of a human being?
If a person falls at terminal velocity, and tries to steer himself with his limbs, what is the maximum lift to drag ratio he can achieve, and what position would that be in?

Guys, there's no need to add words before "lift to drag ratio". Not anymore than there is when asking about an actual aircraft

Even in case of an actual aircraft, say a glider, it is arguably more correct to say that the best L/D ratio or maximum L/D ratio is (say) 40/1, rather than simply to say that the "L/D ratio" is 40/1.  That's the whole point of the "polar curve" graph, to show how L/D continually varies with airspeed (angle-of-attack).  I didn't edit your question (title), and maybe it's a bit overstepping the norm to make an edit along these lines, but technically speaking the edit did make the (title) more correct.

I'm an ex skydiver. It's a position known in skydiving as a "Flat Track", and is more or less the same configuration as a ski jumper takes. Legs together (ideally), bent forward at the hips slightly or flat, arms at the sides, palms flat. You're trying to turn your body into a "lifting body" type aircraft.

In skydiving videos, it's the position you see jumpers taking to get away from each other when it's time to deploy their parachutes and they need to cover a lot of ground. There is another position called a "Delta Track" where you keep your legs apart and relaxed. Horizontal speed is about half of a flat track.
The horizontal velocity achievable while flat tracking is about 60 mph (making it quite a dangerous maneuver near others if you don't know what you're doing).
With a vertical terminal velocity of about 120 mph, which drops to around 80-90 mph while tracking, this makes the unenhanced human body's L/D while tracking, at 50-60 mph horizontally, at about 0.6/1 to 0.7/1.  Adding a Wingsuit improves the L/D to better than 1/1, maybe approaching 2/1.
Everything you wanted to know about tracking here.

Are you sure glide ratio (or descent gradient) is a direct expression of L/D?

Presumably the vertical terminal velocity in "flat track" is significantly less than maximum possible t.v. -- (which would be in what posture, head straight down?).  Many readers may wonder about this, so maybe consider adding that info (ballpark number for that t.v. value) as an additional slight enhancement to this great answer?

@Max yes they're exactly equal, see https://aviation.stackexchange.com/q/80731/4108

I wonder how much of that 120mph you could lose if you got the flare just right :)

Actually I had overlooked that point.  The vertical velocity will be less than the normal 120.  A wiki article says 90 vertical. I was trained that the horizontal speed approaches 60 to get the idea across how dangerous it can be.  So it gives an L/D of .66 not .5.  Thanks for the comment.

@DaveGremlin, not enough.  People have made safe landings with just a wingsuit; nobody has safely landed without one.

@Mark 9 is the guy landing in the boxes the real deal? I always thought that was a hoax.

What is all the bulk at his ankles? Is that the "between the legs" part of a wing suit?

It's not a wing suit.  It may be material added at the bottom of the legs to provide some stability, like fins sort of.  Back when I was jumping, super baggy jumpsuits were all the rage, because they were supposed to reduce terminal velocity to about 100 mph.

@Mark People have come pretty close.  Just a sprain, for instance.  But that relies heavily on a good landing area and luck.

@JohnK No, it was a real thing.  But it was set up very carefully, and was still astonishingly dangerous!


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Why do all commercial aircraft have carpets?
When flying I noticed that all the commercial aircraft I have been on use carpets as a floor covering. But why? If you fly in an older aircraft it looks worn out and it is susceptible to stains etc. Why do they not use one of the hard flooring options like linoleum?

I'd guess that carpet patterns on planes, like upholstery on other forms of transport (e.g. train seats) are likely designed to make staining less obvious. Here's a good BBC article about train seats, which have some similar constraints: http://www.bbc.com/autos/story/20160804-why-are-trains-seats-so-hideous

Comfort and vibe; carpeting is the last remaining thing that separates a plane from a school bus with wings. A cheap passenger-train style hard floor on a plane would suck the last remaining bits of hope out of the whole experience of flying somewhere, fully immersing passengers in soulless despair. shudder

I once took a domestic Chinese flight where the floor was grooved rubber matting. Definitely felt cheap and "school bus with wings"!

@jonrsharpe holy cow at the "skin-crawling video" linked from article - and we sit on those things....

Hey, it's just because -be honest, who doesn't like a flying carpet?

helps prevent slips and falls
contains spills (your spilt soda won't ruin your neighbors bag)
more easily removed and replaced
helps reduce cabin noise

I'd guess it's less poisonous in a cabin fire, too.

@ceejayoz That would depend on a lot of things like the material of the carpet, the glue used and what type of hard flooring you are comparing it to.

You may also consider weight

The second bullet point is also even more important than usual on an airplane, as they tend to spend a rather significant amount of time with the floor not perpendicular to the gravity vector (meaning spilled things would spread faster than normal and not necessarily just to your neighbor.)

@reirab in that case it also has a non-slip effect -- even a plastic bag with any weight in it doesn't slide around in normal operations on carpet, but on a shiny surface everyone would end up with bags sliding into their feet on acceleration.

I doubt the "more easily removed". That doesn't depend on the walking surface, only on the backing surface and that can be made identical.

@reirab actually, the only times in flight that gravity isn't pointing straight down, normal to the floor, would be during takeoff and landing. Banked turns merely increase the acceleration felt downwards, they don't change its direction at all.

@grkvlt Well, actual gravity always points toward (a close approximation of) the center of Earth during flight, though you're right that the normal force during turns points more-or-less towards the floor during turns. However, most aircraft fly with a slight nose-up attitude (and the exact angle changes during flight as fuel is burned off and the weight changes.) And, of course, during climb and descent, the angle between the floor and the gravity vector is even more exaggerated. Also, if a turn isn't quite coordinated, then there will be also be apparent lateral forces.

The answer: High heels!
The local pressure of high heels would punch a hole in the aluminium or composite flooring, and a carpet is the lightest way to distribute the loads such that the local pressure from high heels can be tolerated. This is the real reason.
From Aircraft Structures Summary

On the floor of the fuselage also very high localized loads can occur,
  especially from small-heeled shoes. Therefore floors need a strong
  upper surface to withstand high local stresses.

Other reasons are improved noise absorption and damping of vibrations. Nobody likes it when the floor panels start to resonate when excited by  some engine or pump frequency.
With composite panels now in widespread use for cabin interiors, manufacturing flaws need to be covered, and carpets serve this purpose well. From Compositesworld.com:

Because sandwich panel facesheets are very thin, core print-through is
  common. Further, pinholes can form as phenolic resins outgas during
  reaction and cure. Therefore, panels visible to passengers typically
  are surfaced with some type of decorative material to cover surface
  flaws. Floor panels can be painted or covered with flame-retardant
  carpet.

skydiving in a Cessna 206 taught me that it's totally fine when bits of the floor and interior resonate, but I can see why it wouldn't be great on commercial. I'd never thought about heels on a plane, who does that!!

Last week I flew on the Airbus A350 demo aircraft which has section of the flooring made out of fake wood. They said it was new for the A350, looked really cool, Qatar Airways has it in their A350 at the entry:
http://www.nycaviation.com/2016/01/photo-tour-onboard-qatar-airways-a350-900xwb/39773
So there must be other reasons for using high heals than local pressure alone.

I’ve spent hours reading your answers (you should write a book) and this is the most surprising!

Could you also imagine the difficulty of holding the trolley during turbulence or anything else? It would roll down and run someone over!
I also saw a Quora post where someone had pointed out it's also flame retardant, which is a good point.
Comfort is important too, not to mention another place to put advertising on some planes

so you do away with trolleys. Who needs meal and drinks service in a run down city bus style airliner anyway :)

Flammability is a rule https://www.law.cornell.edu/cfr/text/14/appendix-F_to_part_25

"The average burn length may not exceed 8 inches, and the average flame time after removal of the flame source may not exceed 15 seconds. Drippings from the test specimen may not continue to flame for more than an average of 5 seconds after falling." that's actually quite interesting

It's significantly easier to print advertising on a hard floor than a carpet, so that's not a contributing factor.

I wouldn't say significantly, given that there are plenty of solutions that all work the same way for both hard and soft floors. I'm not saying replace the entire carpet each time obviously..

Carpet is more pleasant
Aside from the practial reasons Greg already mentioned, most if not all people, will consider a carpet as a more pleasant material than ugly linolum. The fact that you have to replace a carpet after some years, counts also for most the interior in a plane (seats, board-entertainment, cover panels and so on). Furthermore a lot of people take their shoes of during long flights and a carpet feels in this situation a lot more comfortable.
In the good old days TWA rolled out a red carpet for their passengers at the check-in desk :)

The mental image of an endless array of people each enjoying the feel of the same patch of carpet on their shoeless feet is not a pleasant one, in my opinion. :)

I take regulary showers!


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Landing with Gusts
When landing in gusty conditions, many people use half flaps to come in at a higher speed. If you were to use full flaps and a normal approach speed, could a strong gust cause the plane to pitch up violently due to the fact that lift has been increased?
My instructor said you could exceed critical the AOA, is this possible?

Though an accurate answer would rely on knowing what aircraft type you are referring to, I think in general you are also getting a little confused with the idea of lift and pitch. Gusty conditions can cause sudden rises and falls, due to varying relative airflow over the wing. But such a rise does not necessarily mean an increase in pitch, rather the whole aircraft rising, like a ballooning effect*. If the wind dies down after a gust, a sinking down might occur.
*A similar effect experienced when deploying the flaps - increasing the amount of flap used
A higher approach speed with lower flaps increases controllability. The higher speed itself gives increased control authority for the ailerons, elevators and rudders. The lower flap setting reduces sensitivity to lift changes during the gusts.
With this AOA issue you refer to, I wonder if your instructor was referring to a go-around. In case a go-around is necessary, a reduced flap setting allows for a more efficient transition to climb configuration. The aircraft already being at a higher speed reduces the time and distance needed to accelerate to the go-around speed, enhancing safety.

Building on the answer, the higher approach speed creates less vertical lift for the same airflow over the control surfaces. As the lift dissipates and the plane sits down on its landing gear, the transition from aerodynamic-based control to wheel-based control happens with much more overlap. This gives more power to keep the plane tracking straight on the runway.

I'd also like to elucidate that the reason why gusts affect the plane less with flaps up is because they are proportionately a smaller fraction of the overall airspeed. Since lift is a function of airspeed squared this means that a gust will generate substantially less additional lift-- and thus ballooning-- at higher airspeeds.


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Why do older pistons crank for a while before starting?
Watching older planes startup I noticed they crank for a little bit before starting. Why do they also shoot out flames when starting? Mostly in Mustangs and Spitfires.

"shoot out flames", "Spitfire" Hmm...

Aircraft engines are started by priming instead of using a choke. The priming system injects fuel right at the intake ports. On a fuel injected engine, the fuel injectors themselves are used for priming.
On a carbureted engine, or an engine with a pressure carburetor like the Merlin (equivalent to throttle body injection in a car), there will be dedicated priming injectors at the intake ports that are only used for priming, and they can be pressurized either by an electric pump or a manual syringe type hand pump.  The Merlin has a priming system operated by an electric pump with a priming toggle switch.
The priming process sprays a quantity of fuel which coats the intake passage in the vicinity of the primer, just upstream of the intake port and takes some time to evaporate. The evaporating fuel is added to the fuel being injected, or being metered at the carburetor, to provide a mixture that is much richer even than the engine running normally with the mixture setting at full rich, enough to keep a cold engine engine running for 30 seconds or so.  In really cold weather, prestart priming may not be enough and you may need to operate the primer after starting to keep it running without dying right away.
A choke on a carbureted car does the same thing by restricting the intake and skewing the carburetor toward extra rich, for starting and for driving operability while the engine is cold, because you want to drive it right away after starting.
Aircraft engines don't use chokes because of the risks of having a potential obstruction device in the intake, and most of the time they only need enough enrichment to get going initially, because unlike a car they aren't going to be called to do any real work until warmed up.
On bigger engines with lots of cylinders, you want to have all cylinders charged with the priming fuel for the smoothest start, BEFORE you introduce ignition. So you will crank it for two crank rotations before switching on mags, counting 6 blades or 8 blades depending on the prop. Depending on the engine, you may run the primer while cranking, or before cranking.
When you do this, the cylinders that got charged first, because they were on the intake stroke when you engaged the starter, end up completing a full 4 stroke cycle, passing some of the priming raw fuel charge out the exhaust valve, so you have a bit of raw fuel in the exhaust port and stack.  On the Merlin, with an exhaust pipe of maybe 12 inches long, this gives the little light show during start. You'll notice it only happens on certain exhaust stacks, the ones that got charged first. If you over prime the engine, by holding the priming switch too long, you tend to get a bigger flames-and-light show. You're just holding a switch and counting off one-thousand-two-thousand, or whatever it is, in your head, so it's not hard to overdo it.
On a warm engine, there's no need to prime and you normally won't see any flames, or just a hint of flames, during start.
On large radials, the exhaust system ducting is much longer and priming fuel is usually consumed before it exits the exhaust pipe, and you see less flame or none at all, during start. Unless of course the pilot over primes and you get the nice light show as well.

And there I thought they just did it because it looks and sounds really cool.

I’m a “too much information” kind of guy, in that I really appreciate answers that provide a deeper intuition and not just “the answer.”  Thanks for this.

On many aircraft engines, there is a significant distance between the carburetor and the intake valves, so it takes several turns of the crank to get the fuel-air mix all the way out to the cylinders through the induction pipes.
In addition, you want all the cylinders in the engine to be charged like this before you turn on the mags. This requires at the very least that the crankshaft be turned through two complete rotations before mags on both. You monitor this by counting the number of propeller blades that pass by the windscreen as you crank. Four blades on the prop x 2 revolutions = 8 blade passages, then mags on.
Lastly, note that two complete turns after the mix has made it to the cylinders will put some air/fuel mix into some of the exhaust stacks by the time you turn the mags on, and when the engine catches the hot exhaust will set that on fire inside the stack- and flames will then come shooting out.

Just to mention that WW II inline-V engines, used in mustangs and spitfires, had a gearbox, giving crankshaft and propeller different rotation speeds. The four-stroke operation also takes - worst case - one and a halve turn of the crankshaft until a specific cylinder gets its fuel-air charge.